How to fix error error: (-215:Assertion failed) size.width>0 && size.height>0 in function 'cv::imshow' when trying to capture a video stream?

I use the simplest code: import cv2 import time import numpy as np cv2.namedWindow("video") cap = cv2.VideoCapture(1) time.sleep(3) while cv2.waitKey(1)!= 30: flag, frame = cap.read() cv2.imshow("video", frame) cap.release() cv2.destroyAllWindows() and I have an error: (-215:Assertion failed) size.width>0 && size.height>0 in function 'cv::imshow' How can I fix that? Thanks